Production of acetic acid from methanol



United States Patent PRODUCTION OF ACETIC ACID FRGM Mil-ETHANOL EdwardBoaden Thomas, Thomas Hall Stothard, and Edmund Harry Alcock, Spondon,near Derby, England, assignors to British Celanese Limited, acorporation of Great Britain No Drawing. Application October 22, 1952,Serial No. 316,318

11 Claims. (Cl. 260-532) This invention relates to the production oforganic compounds and is concerned, more particularly, with theproduction of acetic acid by reaction between carbon monoxide andmethanol.

U. S. patent application S. No. 188,481, filed October 4, 1950, nowPatent No. 2,659,246 describes a process for the production of aceticacid by reaction between carbon monoxide and methanol in a reaction zonecontaining active carbon impregnated with nickel iodide. The saidapplication shows how the process can be carried out continuously in thevapour phase without such loss of nickel and iodine from the reactionzone as to render the process commercially unattractive and withoutmeeting serious ditiiculties due to corrosion of equipment. in thisprocess there is, however, some loss of catalytically active materialfrom the reaction zone in the gaseous efiluent, so that for continuousoperation over long periods it is necessary to provide for the supply ofboth nickel and iodine to the reaction zone.

According to the present invention, acetic acid is produced by acontinuous process which comprises reacting carbon monoxide withmethanol in the vapour phase in presence of active carbon impregnatedwith nickel iodide as catalyst, the carbon monoxide being passed throughsuccessive zones of active carbon and the iodine content of the catalystbeing maintained by a suitable supply to the reactor, and periodicallyreversing the direction of how of the reactants, the methanol beingintroduced so that it by-passes the first zone. of active carbon in thedirection of flow obtaining and the last zone of active carbon in saiddirection being maintained at a temperature above that at which aceticacid condenses but below the reaction temperature so that it absorbsnickel from the outgoing substances without condensation of acetic acidand, upon reversal of flow, yields nickel to the incoming gas.

it has been found that by operating in this manner it is possible withcareful control of the reaction conditions to avoid the loss of nickelfrom the reaction zone or to reduce this loss to such very smallproportions that the process can be operated continuously for severalweeks without supplying nickel to the reaction zone during the process.

The reactor employed for carrying out the process of the invention mayconsist of a single vessel comprising a number of zones which may beseparated, for example by means of grids. At each end of such a vesselthere is an end zone of active carbon for the purpose of alternatelyabsorbing nickel from and giving nickel up to the gas passing throughthe zone and, immediately beyond the first end zone in t. e direction offlow of the reactants and immediately in front of the second end zone,are mixing zones. The methanol is introduced into these mixing zonesalternately according to the direction of flow of the reactants so as tomeet and mix with the carbon monoxide being introduced through theadjacent end zone. The middle part of the vessel contains active carbonimpregnated with nickel iodide and forms the reaction zone.

"ice

Provision may be made for introducing the methanol at one or more pointsalong the reactor in addition to the points of introduction adjacent theend zones and, if desired, at each point where methanol is to beintroduced, i. c. at each mixing zone, the vessel may contain a layerofinert packing material, e. g. glass or porcelain beads, pebbles ormaterials of higher thermal conductivity, e. g. graphite granules, sothat in effect several shallow reaction zones are employed in series. 5,10 or even more, e. g. up to 15 or 2%, such shallow reaction zones maythus be employed, preferably with end zones which are of substantiallygreater depth than, e. g. 2 or 3 times the depth of, the individualshallow reaction zones. The depth of the end zones in relation to thetotal reaction zone depth depends upon the rate of passage of thereactants, the reaction conditions, and the temperature at which the endzones are maintained. The depth of each end zone may be greater than thetotal reaction zone depth, for instance up to twice this depth, but maybe less, and satisfactory results have been obtained by using end zoneseach equal to a quarter of the total reaction zone depth. Where themixing zones are constituted by layers of inert material such layers canbe of much less depth than the total reaction zone depth, and where asuccession of shallow reaction zones are employed can be of less depththan each such reaction zone provided they are deep enough to ensureevaporation of any liquid supplied to them and thorough admixture of thevapour formed or introduced with the reactants issuing from thepreceding zone; the achievement of the latter result is much assisted bythe use of eflicient means for dispersing the material introducedthroughout the zone.

instead of employing a single vessel comprising a number of zones,several vessels may be employed in series, each vessel comprising one ormore zones. Thus, a pair of vertical tubular vessels joined at the topmay be employed so that the incoming carbon monoxide is alwaysintroduced in an upward direction and the issuing products are alwaystaken ofi in a downward direction. The same result may be achieved byusing a reaction vessel in the form of an inverted U. If desired, theend zones may be accommodated separately.

The reaction vessel or vessels may be jacketed and/or provided withinternal tubes and means provided for circulating an attemperatingmedium through such jacket and tubes. In this way provision is made bothfor raising the temperature of the reactor when starting up the reactionas Well as for effecting the cooling needed during the progress of thereaction.

When employing a number of successive reaction zones separated by mixingzones, a very efiective method of providing cooling consists insupplying liquid methanol to the mixing zones. The methanolevaporates'in the mixing zones, cooling the gases issuing from theimmediately preceding reaction zone, so that as the reaction gasesprogress through the reaction vessel their temperature rises and fallsas they pass successively through reaction zones and mixing zones. It ispreferred to provide the feed pipes supplying the methanol with externalcooling jackets reaching within a short distance of the reac tionvessel, and to provide means within the reaction zones, e. g. a sprayhead or a spider of perforated pipes, to ensure distribution throughoutthe mixing zones of the methanol supplied by the feed pipes.

Provision may be made for Withdrawing the reactant and product mixturefrom the reactor, passing it through a cooler and then returning it tothe reactor, the points of exit and entry being such that the mixturethus recycled does not pass through either of the end zones. By adoptingthis expedient maintenance of a steady reaction temperature is possibleeven Without any other cooling means being provided, especially if thequantity of gas recycled is large in relation to the quantity of freshgas passed into the reactor through the end zone, e. g. more than timesand preferably equal to about 8 to 12 times the volume of the fresh gas.Acetic acid may, if desired, be condensed from the circulating gasbefore it is returned, but in this case provision must be made forreheating the gas to a temperature suitable for reintro duction, e. g.to C. below the desired reaction temperature.

The impregnation with nickel iodide of the active carbon employed canconveniently be carried out soaking the active carbon in an aqueoussolution of nickel iodide and drying the product until its water contentis reduced to a suitable value. sults are obtained using a productcontaining not more than 6 or 7 molecules of water per molecule ofanhydrous nickel iodide, but much less water than this may be present,and if desired, a substantially anhydrous catalyst may be used. It isnot necessary that the active carbon should carry a high proportion ofnickel iodide, and quantities equal to /2 to 1 gram molecule of nickeliodide per litre of granular active carbon are adequate and even smallerquantities may be used. The active carbon used is preferably a highlyabsorptive type graded 8 to, 10 mesh. The impregnated carbon granulesmay be diluted with a material inert to the reactants and products,especially such a material which is of good thermal conductivity, e. g.graphite granules. Usually such a diluent is not used in greaterquantity than 2 or 3 parts by volume per volume of impregnated activatedcarbon granules. 7

Generally satisfactory re- 7 In starting up the process carbon monoxidealone cry in admixture with hydrogen, water vapour or inert gases ispassed into the reaction vessel through an end zone to meet methanolintroduced immediately beyond this zone- Where a succession of reactionzones are used additional methanol can be introduced into some or all ofthe subsequent mixing zones. The carbon monoxide may be such as toprovide the theoretical quantity needed to react with all the methanolto be supplied but is preferably more than this, especially whenhydrogen is present. The gas fed in may be preheated and the Wholereaction vessel may be raised to a temperature suitable for initiatingthe reaction before any reactants are introduced. During the process theend zone being used to absorb nickel from the gas passing through italso absorbs some but not all of the iodine carried by the gas.Accordingly the iodine content of the catalyst is maintained by asuitable supply to the reactor. In practice it is preferred to introducethe iodine in the form of methyl iodide, conveniently together with themethanol.

Where the reactants pass through a succession of reaction zones methanolmay be introduced at each of the mixing zones and may be supplied inliquid form to absorb heat generated in the preceding reaction zone andto provide for continuing the reaction in the succeeding zone. Ifdesired, however, methanol may be supplied only to the earlier mixingzones, say the first third or half of the mixing zones, in quantity suchthat the reaction tends to fall ofi thereafter, this eifect beingassisted, if necessary, by suflicient dilution of the methanol withwater. The nickel tends to travel along the reaction zones in thedirection followed by the reactant stream and the site of the mainreaction moves with the nickel; after a While it becomes desirable tostart introducing methanol at the first zone (counting in the directionof flow) into which no methanol has so far been introduced and, at thesame time, it is possible to reduce or stop the supply of methanol tothe very first mixing zone. This operation 1 can be repeated untilmethanol is being introduced into the mixing zone preceding the lastreaction zone. It is then time to reverse the direction of flow andaccordingly the direction of travel of the side of main reaction which,it will be appreciated, never occupies more than a third to half of thereaction zones. Where a single zone of otherwise be carried out of thereactor by the gaseous eflluent. By the provision of thermo-couples thesite of the main reaction can always be located, whether one or aplurality of reaction zones be employed.

Methyl iodide can be recovered from the products of the process byfractionation inquantity varying according to the reaction conditionsand can be recycled. Another product of the process which can withadvantage be recycled is methyl acetate. It has been found that byrecyclin the methyl acetate and introducing it together with methanol,the ratio of acetic acid to methyl acetate obtained in the process canbe increased. As already indicated, water may also be introducedtogether with the methanol and, indeed, the presence of water in thereactants assists in suppressing the production of methyl acetate.However, it has been found that the introduction of large quantities ofwater is liable to lead to increased loss of iodine from the reactionvessel. It is usually found that the use of 5 to 10 moles of methanol,and especially. about 8 moles of methanol, for each mole of water givessatisfactory results, although where methyl acetate is introduced intothe reaction vessel with the methanol somewhat more water can be used,for instance an additional-quantity of up to 1 mole per mole of methylacetate thus introduced.

The temperatures and pressures employed in carrying out the process ofthe invention can be substantially'below those usually recommended asbeing best for the synthesis of acetic acid from carbon monoxide andmethanol and, in particular, it is preferred to use temperatures notsubstantially above 250 C. and pressures below 50 atmospheres. Indeed,the process can be carried out to give commercially useful conversionsand yields using temperatures between and 250 C. and pressures below 30atmospheres. In general it is preferable to use temperatures in theneighbourhood of 200 C. to 230 C. at pressure of 10 to 30 atmospheres;higher outputs can be achieved at higher temperatures but it is usuallybest to avoid temperatures much above 260 C. Under these conditions theend zones can be maintained at temperatures within the range of to C.

By using low pressures low reaction temperatures can be employed withoutthe acetic acid produced condensing in the reaction vessel. corrosion ofequipment and leads to washing of the active carbon with the condensedacid and thus removal of nickel and iodine compounds from the reactionvessel where a downwardly travelling reaction mixture is employed andrefluxing of the acetic acid where an upwardly travelling reactionmixture is employed; these effects are most undesirable. Moreover, theuse of low temperatures makes possible the carrying out of the processin a presence of hydrogen without the excessive formation of methanewhich has been found to occur in the presence of hydrogen attemperatures considerably above 200 C.,

' especially at the temperatures usually recommended for the productionof acetic acid from carbon monoxide and methanol, i. e. 300 to 400 C. Inthis way the acetic acid can be produced directly fromv the productsobtained by reacting carbon monoxide and hydrogen in the presence of amethanol-forming catalyst without first separating the methanol.Alternatively, the methanol can be subiected to reaction with carbonmonoxide using water gas as the source of carbon monoxide, the water gasbeing thereby enriched in hydrogen and made suitable for subsequent usein the synthesis of methanol.

"However, the invention is not limited to the use of relatively lowtemperatures and pressures and tempera- Such condensation causes severetures of up to 300 or 400 C. may be used, particularly where the processis carried out in the absence of hydrogen. Temperatures above 200 C., e.g. up to 250 or even 260 C., can be used even with hydrogen-containingreactants Where the higher conversion is regarded as of suflicient valueto offset the lower yield based on the methanol used which results fromthe greater methane formation. Pressures of more than 50 atmospheres, e.g. 100 atmospheres, and even much higher pressures, e. g. 200 or 300atmospheres, can be used and the size of the apparatus for a givenoutput thereby reduced but, in general, there seems to be littleadvantage in using very high pressures.

The contact time employed in carrying out the process of the inventioncan be varied within quite a wide range, and continuous operation hasbeen carried out using contact times (based on the reactants being fed)of substantially less than 1 minute, e. g. 0.1 to 0.2 minute, althoughlonger times, e. g. l to 3 minutes, are preferred and even longer times,e. g. 5 or more minutes, may be used if desired. As already indicated,the direction of fiow of the carbon monoxide is repeatedly reversedduring the process of the invention. With a contact time of l to 3 minutes at a temperature in the range 190 to 250 C. and a pressure in therange to 30 atmospheres, it is preferred to reverse the flow after aninterval of time of at least 2 hours, preferably after 4 to 8 hours. Thesize of the end zones must, of course, be related to the operating cycleso as to avoid them becoming so charged with nickel that they no longerextract it efficiently from the gases and vapours passing through them.

Under the conditions which are preferably employed for carrying out theprocess of the invention, i. e. at a temperature not substantially above250 C. and a pressure below 30 atmospheres, nickel chloride and nickelfluoride and the halides of other metals, including the other ironmetals, show little or no catalytic activity while only active carbonappears to be useful in the process, other substances often used ascatalyst supports giving results which are useless from a commercialpoint of view.

The following example illustrates the invention:

Example The process is carried out using an upright jacketed reactorcharged with three-inch deep layers of catalyst separated from eachother by 14 two-inch deep layers of porcelain beads. The catalyst andbead layers (which form the reaction zones and mixing zones) are eachsupported on perforated baffles and the end catalyst layers areseparated from 15 inch deep end zones charged with unimpregnated activecarbon. Thermocouples are provided in the catalyst layers and in the endzones. The catalyst consists of active carbon granules impregnated withone mole of nickel iodide per litre of granules.

A jacketed pipe leads through the walls of the reactor into each mixingzone where it feeds a distributor arranged to disperse liquid suppliedthrough the pipe throughout the layer of beads. The reactor is providedwith means for the circulation of a heating or cooling medium. 7

In operation, the reactor, apart from the end zones which have separateattemperating systems, is heated to a uniform temperature of 200 C. Theend zones are initially heated to 170 to 180 C. and each in turn ismaintained within this temperature range throughout operation bycirculating a cooling medium. When stable temperature conditions areattained a mixture of carbon monoxide and hydrogen, preheated to 200 C.,is introduced through one end zone while at the same time aqueousmethanol containing some methyl acetate and a little methyl iodide isintroduced through the first three jacketed pipes, cooling Water orbrine being circulated through the jackets to keep the mixture liquid.

The temperature throughout the reactor rises somewhat, that of the firstthree or four reaction zones attaining a temperature of 240 to 250 C.while the rest of the reactor only attains a temperature of 220 to 230C. or even lower. After some time the region of highest temperaturestarts moving in the direction of travel of the reactants and,accordingly, the jacketed feed pipes in use are from time to timealtered until only the last four pipes are in use. The direction of flowshould be changed before the last reaction zone rises to the highesttemperature; under the conditions described reversal every five hours issatisfactory. With the change in the direction of flow the end zone atwhat had been the output end of the reactor is allowed to rise intemperature under the influence of the hot incoming reactants which pickup adsorbed nickel and iodine compounds; the other end zone is meanwhilecooled to to C. and operates at this temperature until the next flowreversal.

During the operation attemperating medium can be continuously circulatedthrough the jacket of the reactor but the medium should be maintained ator a little below 220 0, say between 200 and 220 C.

In the event of a thermocouple showing the development of an undesirablyhigh temperature in any particular zone, the rate of feed or content ofwater of the methanol fed to the preceding mixing zone is increased andlikewise a fall in temperature can be corrected by a suitable reductionin the feed to that mixing zone.

The operation is carried out using a pressure of 300 lbs. per squareinch and a total throughput of 44 moles of water, 17.5 of methylacetate, 1.1 of methyl iodide, 180 to 200 of carbon monoxide and to 220of hydrogen per 100 moles of methanol supplied at a rate giving acontact time of 2 to 3 minutes based on the reactants (i. e. carbonmonoxide and methanol) fed. The process can be continued for a period ofmore than 600 hours before the conversion of methanol to acetic acidfalls below 50%, the conversion during the earlier part of the processamounting to 60 to 65%. During the 600 hours the whole of the aceticacid formed is recovered as free acetic acid but during the succeeding100 hours a small quantity of methyl acetate is formed in addition. Theyield of total acetic acid based on the methanol used falls from a valueof 65% to somewhat below 50% after 700 hours.

Having described our invention, what we desire to secure by LettersPatent is:

1. A continuous process for the production of acetic acid, comprisingreacting carbon monoxide with methanol in the vapour phase in presenceof active carbon impregnated with nickel iodide as catalyst, the carbonmonoxide being passed through successive zones of active carbon and theiodine content of the catalyst being maintained by a suitable supply tothe reactor, and periodically reversing the direction of flow of thereactants, the methanol being introduced so that it by-passes the firstzone of active carbon in the direction of flow obtaining and the lastzone of active carbon in said direction being maintained at atemperature above that at which acetic acid condenses but below reactiontemperature so that it absorbs nickel from the outgoing vapours withoutcondensation of acetic acid and, upon reversal of flow, yields nickel tothe incoming gas.

2. Process according to claim 1, wherein the iodine content of thecatalyst is maintained by introducing methyl iodide recovered from theproducts of the reaction.

3. Process according to claim 1, wherein water is introduced into thereactor during the process.

4. Process according to claim 1, wherein methyl acetate is introducedinto the reactor during the process.

5. Process according to claim 1, wherein a reaction temperature ofbetween 200 and 260 C. is employed and a pressure of less than 50atmospheres.

6. Process according to claim 1, wherein a reaction time of 1 to 3minutes is employed and the direction of flow is reversed at intervalsof 4 to 8 hours.

7. Process according to claim 1, wherein the reactants pass throughsuccessive zones of active carbon impreg mated with nickel iodide andliquid methanol is introduced between such zones. 7

.8. A continuous process for .the production of 'acetic acid, comprisingreacting carbon monoxide with methanol in the vapour phase in presenceof active carbon impregnated with nickel iodide as catalyst, the carbonmonoxide being passed through successive zones of active carbon at sucha rate as to give a reaction time of l to 3 minutes and the iodinecontent of the catalyst being maintained by a suitable supply to thereactor, and reversing the direction of flow of the reactants atintervals of 4 to 8 hours, the methanol being introduced so that 'itbypasses the first zone of active carbon in the direction of flowobtaining and the last zone of active carbon in said direction beingmaintained at a temperature above that at which acetic acid condensesbut below reaction temperature so that it absorbs nickel from theoutgoing vapours without condensation of acetic acid and, upon reversalof flow, yields nickel-to the incoming gas.

9. A continuous process for the production of acetic acid, comprisingreacting carbon monoxide with methanol in the vapour phase at atemperature between 200 and 260 C. and under a pressure of less than 50atmospheres in presence of active carbon impregnated with nickel iodideas catalyst, the carbon monoxide being passed through successive zonesof active carbon at such a rate as to give a reaction time of l to 3minutes and the iodine content of the catalyst being maintained by asuitable supply to the reactor, and reversing the direction of flow ofthe reactants at intervals of 4 to 8 hours, the methanol beingintroduced so that it by-passes the first zone of active carbon in thedirection of flow obtaining and the last zone of active carbon in saiddirection being maintained at a temperature above that at which aceticacid 10. A continuous process for the production of acetic acid,comprising reacting carbon monoxide and methanol in the vapour phase ata temperature between 200 and 260 C. and under a pressure of less than50 atmospheres in presence of water and methyl acetate and of activecarbon impregnated with nickel iodide 'as catalyst, the carbon monoxidebeing passed through successive zones of active carbon at such a rate asto give a reaction time of 1 to 3 minutes and the iodine content of thecatalyst being maintained by a suitable supply .to the reactor, andreversing the direction of flow of the reactants at intervals of 4 to 8hours, the methanol being introduced so that it by-passes the first zoneof active carbon in the direction of flow obtaining and the last zone ofactive carbon in said direction being maintained at a temperature abovethat at which acetic acid condenses but below reaction temperature sothat it'absorbs nickel from theoutgoing vapours without condensation ofacetic acid and, upon reversal of flow, yields nickel to the incominggas.

11. Process according to claim 10, wherein the reactants pass throughsuccessive Zones of active carbon impregnated with nickel iodide andliquid methanol is introduced between such zones.

References Cited in the 'file of this patent UNITED STATES PATENTS GreatBritain Aug. 27, 1930 V

1. A CONTINUOUS PROCESS FOR THE PRODUCTION OF ACETIC ACID, COMPRISINGREACTING CARBON MONOXIDE WITH METHANOL IN THE VAPOR PHASE IN THEPRESENCE OF ACTIVE CARBON IMPREGNATED WITH NICKEL IODINE AS CATALYST,THE CARBON MONOXIDE BEING PASSED THROUGH SUCCESSIVE ZONES OF ACTIVEMONOXIDE AND THE IODINE CONTENT OF THE CATALYST BEING MAINTAINED BY ASUITABLE SUPPLY OF THE REACTOR, AND PERODICALLY REVERSING THE DIRECTIONOF FLOW OF THE REACTANTS, THE METHANOL BEING INTRODUCED SO THAT ITBY-PASS THE FIRST ZONE OF ACTIVE CARBON IN THE DIRECTION OF FLOWOBTAINING AND THE LAST ZONE OF ACTIVE CARBON IN SAID DIRECTION BEINGMAINTAINED AT A TEMPERATURE ABOVE THAT AT WHICH ACETIC ACID CONDENSESBUT BELOW REACTION TEMPERATURE SO THAT IT ABSORBS NICKEL FROM THEOUTGOING VAPORS WITHOUT CONDENSATION OF ACETIC ACID AND, UPON REVERSALOF FLOW, YIELDS NICKEL TO THE INCOMING GAS.